Switchable coaxial probe member utilizing a reed switch

ABSTRACT

A COAXIAL CABLE IN THE FORM OF A PROBE INCLUDES AN OUTER ENCLOSING CONDUCTOR SUBSTANTIALLY CONCENTRIC WITH A CENTRAL CONDUCTING MEANS. THE CENTRAL CONDUCTING MEANS INCLUDES A REED SWITCH, E.G. FOR CLOSING AND OPENING THE PROBE CIRCUIT OR FOR CONNECTING A RESISTANCE IN SERIES THEREWITH. AN ACTUATOR, AXIALLY SLIDABLE ON THE OUTER CONDOCTOR, INCLUDES A MAGNET FOR OPERATING THE REED SWITCH WHEREBY SWITCHING MAY BE ACCOMPLISHED WITHIN THE PROBE WITHOUT AN INTERVENING MECHANICAL LINKAGE.

` Feb; 2,.1971 M. D. BUTLER I I 3,560,848

Y ASAWITCHABLE OAXIAL PROBE MEMBER UTILIZING A REED SWITCH Filed May :20, 1968 Buc/(Hom, BLORE, KLAnou/sr a 'SPAR/MAN Arromfrs MARLow D. BUTLER 3,560,848 SWITCHABLE COAXIAL PROBE MEMBER UTILIZING A REED SWITCH Marlow D. Butler, Portland, Oreg., assigner to Tektronix, Inc., Beaverton, Oreg., a corporation of Oregon Filed May 20, 1968, Ser. No. 730,538 Int. 'Cl. G01r 3] 02 U.S. Cl. S24-72.5 11 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION A coaxial cable is suitably employed for coupling signals having a wide range of signal frequencies from an input device to an output device. Such `a coaxial cable includes a central conductor and an outer shielding conductor concentrically spaced from the central conductor in enclosing relation therewith. The predetermined constant spacing between the inner conductor and the outer conductor is such as to maintain a substantially constant characteristic impedance for the cable. Attempts to connect a switching means in series relation with the cables central conductor have not been altogether successful because of the physical proportions of a switch mechanism in relation to the usual cable. A switching linkage or structure is apt to cause the cable to be shunted with excessive capacitance and, moreover, the switching linkage usually provides a different shunting capacitance in different switching positions thereof.

SUMMARY `OF THE INVENTION According to the present invention, a switchable coaxial cable comprises an outer conductor and an inner conductor substantially coaxial with and enclosed by the outer conductor wherein the inner and outer conductors provide a circuit path for connection to a source of signals at one end, while providing a source of signals at the other. A magnetic reed switch is also located within the outer conductor, the reed switch being provided with a pair of contacts connected in circuit with the inner conductor. An actuator located outside and supported by the outer conductor is adapted for movement relative to the outer conductor and relative to the reed switch. The actuator includes a magnet which is moved by the actuator into operative juxtaposition with the reed switch for closing the contacts thereof, and the reed switch contacts are thus closed and opened without employing mechanical linkage. As a result, the impedance of the coaxial cable is not materially affected. In a probe embodiment of the present invention, the aforementioned actuator is slidably mounted on the cylindrical hand-held probe body and may be conveniently manipulated by a person employing the probe. The reed switch is desirably coupled to connect and disconnect an attentuating resistor which is also located within the probe body It is accordingly an object of the present invention to provide an improved switchable coaxial member having substantially constant impedance properties.

It is a further object of the present invention to pro- United States Patent O vide a switchable coaxial member having low switch capacitance, with substantially no mechanical switch linkage being located within the coaxial member.

It is a further object of the present invention to provide an improved switchable coaxial member having low switch capacitance, hermetically sealed contacts, extremely long life, and which is easy to manufacture.

It is another object of the present invention to provide an improved hand-held coaxial probe having switching means incorporated in the probe, wherein the switching means does not materially alter the desired electrical impedance properties of the probe.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. 1 is a longitudinal cross section of a switchable coaxial member in the form of a probe according to the present invention;

FIG. 2 is a circuit diagram of a probe of FIG. 1; and

FIG. 3 is a circuit diagram of a probe and a cable connected thereto including a circuit for a second reed switch employed with the probe.

DETAILED DESCRIPTION Referring to the drawings, and particularly to FIG. l, a switchable coaxial member according to the present invention in the form of a probe includes a conducting metal probe body having a forward portion 10 and a rearward portion 12 soldered together at 14. The probe body is suitably formed of brass, and may be gold plated. Both the front and rear portions are cylindrical, having an inner bore, with the forward portion 10 being received a short distance within rear portion 12. Rearward portion 12 of the probe body includes an enlarged rearwardmost end 16 provided with an internally threaded socket 18 for receiving cable bushing 20. The internally threaded socket 18 is larger in diameter than central inner bore 22 in rearward portion 12 of the probe body with which it communicates.

The end 16 is enclosed by a cylindrical outer insulating cover 24, suitably formed of plastic, having an inwardly extending radial flange 26 received around a stepped-down shoulder 28 on body end 16. Outer insulating cover 24 extends rearwardly of body end 16 where it partially encloses cable bushing 20, and cover 24 also extends forwardly of end 16 leaving a pocket 32 between body portion 12 and cover 24. The forward portion 10 of the probe body is for the most part enclosed within cylindrical insulating forward body cover 34, which may also be formed of plastic. Body cover 34 is flared to a larger diameter at its rearward end where it is received over rearward body portion 12, having a plurality of gripping teeth 36 at that point for engaging body 34 and holding the latter in place. The forwardmost end of body portion 10 is exposed, and is internally provided with an insulating spacer member 38 for concentrically supporting a conducting metal probe tip 40. Probe tip 40 extends rearwardly through spacer member 38 and into a central extension 42 of spacer member 38 where the probe tip connects to a conducting inner bore in central extension 42, the latter forming a conducting socket or jack for a plug 44 joined to the central conducting means of the probe. The extension 42 is countersunk at 45 to guide plug 44 into the jack thus provided.

Metal cable bushing 20, for joining connecting cable 54 to the probe, includes a central conductor 46 insulated from threaded outer conductor portion 48 by a cylindrical insulator 50. The central conductor 46 is connected to the central conductor 52 of coaxial cable 54, while the outer concentric conductor 62 of cable 54 is engaged by stress-relieving teeth 56 formed on tubular extension 58 of bushing 20. A ferrule 60 clamps the cable upon tubular extension 518. vOuter conductor portion 48 of bushing is theadably received in internally threaded socket 18 of body end 16 until radial flange 30 of the cable bushing abuts body end 16 of the probe within cover 24. Through the threaded connection, the outer conductor62 of the cable 54 is electrically connected to the probe body.

Inner bore 22 of rear body portion 12, which is slightly larger in diameter than the inner bore of forward body portion 10, receives therewithin a substantially cylindrical, spool-like holder 64 which is formed of insulating material, e.g. plastic. This holder is provided on each side with a channel for receiving a reed switch, the holder forming a clip having plastic tabs 100 for holding each such reed switch in position. One such reed switch 66, for example, includes a pair of contacts, formed of conducting and magnetically permeable material, which normally make no contact. However, when the reed switch is located in a magnetic field of predetermined strength, the magnetic lines of ux flowing substantially parallel to the reed switch contacts cause them to become magnetized and attract one another, thereby making contact. Of course, when the magnetic field is removed, the contacts are once more opened. The contacts are enclosed in a small hermetically sealed insulating, e.g. glass, envelope, having a pair of conductors connected to the respective contacts extending from either end thereof.

The reed switch 66 is here connected in parallel with an attenuating resistor 68 and also in parallel with a capacitor 70, the ciruit being represented in diagrammatic form in FIGS. 2 and 3. In the FIG. 2. diagram the components are arranged physically in the same relative position they have in the probe, for ease of comparison. Both attenuating resistor 68 and capacitor 70 are physically located between holder 64 and probe tip 40, within the -bore of forward body portion 10, as can be seen in the FIG. 1 cross section. Capacitor 70 is suitably a ceramic capacitor provided with a longitudinal bore for receiving resistor lead 72, which makes no internal connection to the capacitor, but which extends therethrough where it is connected to the forward end of the capacitor by means of short lead 74. Lead 72 is also joined to plug 44. The remaining end of capacitor 70 is connected to the remaining resistor lead 76, which extends along a slot in holder 64 and connects to central conductor 46 of bushing 20. Reed switch 66 is connected across resistor 68, between lead 76 proximate central conductor 46, and lead 72 between resistor 68 and capacitor 70.

Conductors 76 and 72, resistor 68, plug 44, capacitor 70, reed switch 66, and associated conductors comprise a central conducting means for the coaxial probe within the bore of the probe body. The central conducting means is not physically joined to the interior bore of the probe body except by means of plug 44 and bushing 20. Therefore, the central conducting means including the aforementioned components may be removed with removing bushing 20. Thus, bushing 20 is rotated until its threads no longer engage thread 18 whereupon the central conducting means may be axially withdrawn from the probe. Therefore, the internal wiring of the probe can be easily changed if desired without complete mechanical disassembly of the probe. The central conducting means are just as easily inserted axially from end 16 with plug 44 being guided into the jack provided in extension 42 by the countersunk hole at 45. The bushing 20 is tightened until flange 30 abuts end 16 of the probe body. It is noted that holder 64 centrally positions a reed switch or switches within the inner bore 22 whenever the central conducting means is inserted within the probe.

The probe is further provided with a cylindrical actuator 78 coaxially received around body portion 12, with sufficient tolerance for axial sliding movement thereof with respect to body portion 12. The actuator 78, which is desirably formed of plastic, is provided with radial inwardly extending flanges 80 and 81, near the forward and rearward ends thereof respectively, which slidably engage the outer cylindrical surface of rear body portion 12. Between radial flanges 80 and 81 and within actuator 78 is positioned a cylindrical permanent magnet 82, around which actuator 78 is suitably formed. Longitudinally outward from flanges 80k and 81, actuator 78 extends axially Within pocket 32 at one end, and over the flared portion of cover 34 at the other. At the switchoff position for the magnet, shown in full lines, actuator 78 nearly covers the larger diameter portion of cover 34. The enlarged or flared portion of cover 34 prevents actuator 78 from moving any farther forward because the forwardmost flange 80 is stopped by cover 34. At the switch-on position for the magnet 82 (shown in dashed lines at 82'), the actuator 78 is received further within pocket 3,2, its rearward movement being limited by shoulder 28 of body end 16 and radial ange 26 of cover 24. Magnet 82 has north and south poles at its ends.

Actuator 78 is provided with a plurality of snap latches 98 formed of plastic which are flexible because of their narrow dimension and which extend axially rearward from radial flange 81. The snap latches are suitably extensions of holder 64, being formed at the same time therewith. These snap latches are adapted to engage forward detent groove 94 when the actuator is in its forward most position (shown in full lines), and to engage detent groove 96 when the actuator is in its rearwardmost position. Detent grooves 94 and 96 are circumferentially provided in the outside surface of rearward body portion 12. Thus, two easily discernible switching positions can be recognized by the operator. The actuator 78 is also provided with a plurality of knob grooves 79 which extend substantially circumferentially around actuator 78 except for the bottom at surface thereof 102 where actuator 78 is of decreased radial dimension. These grooves are easily grasped vby the operator in moving actuator 78 in a longitudinal direction. Surface 102 matches a corresponding inner surface 104 inside pocket 32 for keying actuator 78 so that it may move axially, While excessive rotation thereof is avoidied.

Holder 64 suitably carries a second reed switch 88 adjacent and operable at the same time as reed switch 66. Reed switch 88 is suitably identical in construction to reed switch 66 and is provided with a pair of end conductors and 92 which extend through aperture 106 in bushing 20. These conductors may then extend inside the covering and outside the outer conductor 62 of cable 54 to a remote location. As will be appreciated, reed switch 88 will be operated at the same time as reed switch 66 since it is positioned in juxtaposed relation with switch 66. Conductors 90 and 92 are suitably employed to connect to the instrument to which cable 54 is connected for bringing about a visual indication or the like in the instrument, notifying the user that the attenuation provided by the probe has been changed. Reed switch 88 is positioned in a channel in holder 64 opposite the channel in which reed switch 66 is received, and reed switch 88 is similarly held in place by tabs 100 provided on holder 64.

FIG. 3 is `illustrative of the probe circuit, as well as the instrument input circuit to which it may be connected. The probe, including reed switch 66, resistor 68,

and capacitor 70 in parallel, is electrically located between a test point and coaxial cable 54 extending to a measuring instrument, such measuring instrument having an input impedance represented by resistor '86. Thus the probe in series with coaxial cable 54 may be employed for coupling to a source of signals while providing an output at a measuring instrument. In practice, the probe, which is physically small, is hand held, and probe tip 40 is contacted to a selected point in an electronic circuit under test. In accordance with the present invention, the attenuation of the signal delivered to the instrument is conveniently changed by manipulating actuator 78. With the actuator in the position shown vin full lines in FIG. 1, magnet 82 is sufficiently forward, relative to reed switches 66 and 88, so that the contacts of these reed switches remain in their normally open position. However, if the actuator 78 is moved rearwardly to its rearwardmost position, such that the 'magnet is located at the position shown in dashed lines at 82 in FIG. 1, the lines of magnetic flux passing through the cylindrical aperture of the magnet will magnetize the reed switch contacts and cause them to close.

When the contacts of reed switch 66 are open, resistor 68 is serially inserted in the circuit providing a predetermined degree of attenuation of the signal coupled to the measuring instrumentf'In the particular device illustrated, the measuring instrument had an input impedance of one megohm (as represented by resistor 86), and resistor 68 had a resistance of nine megohms. When the reed switch 66 is closed, shunting resistor 68, the amplitude of the signal provided at the instrument is on the order of ten times the amplitude provided with resistor 68 in the circuit. Capacitor 84, which may be included as a part of the instrument, or in a termnial connector of cable 54, is utilized for impedance adjustment. The circuit comprising capacitor 70 in shunt with resistor 68 is arranged to have substantially the same time constant as capacitor 84 in shunt with resistor 86. It will be appreciated that the reed switch can also be employed to perform other functions. Thus, for example, with resistor 68 and capacitor 70 left out of the circuit, reed switch 66 will provide an on-off function.

In the configuration of the probe according to the present invention, the actuator 78 operating the reed switches is easily controlled to close and open the reed switches with the same hand in which the probe is held, thus releasing the other hand for other jobs. Moreover, since the attenuation in the circuit is controllable at the probe, it is not necessary to use the attenuation controls at the measuring instrument itself, which are not generally as close at hand. The operator may thus avoid reaching to the instrument itself to control attenuation. Therefore, the probe according to the present invention enhances the speed and flexibility of instrument utilization.

It is important to note that the switching accomplished in the probe according to the present invention has little effect upon the probe characteristic impedance, which, of course, is designed to match that of cable 54. The reed switches are quite small, and the reed switch contacts move comparatively small distances from an off position to an on position. Moreover, and more important, no mechanical switch linkage extends through the outer coaxial conductor, formed by the probe body, to the inner coaxial conductor or central conducting means within the probe. Therefore, the added capacitance which would arise in electrical shunt with the probe as a result of such mechanical construction is avoided, and a good impedance match is maintained n the probe.

Since the actuator 78 including magnet 82 is entirely external of the outer coaxial conductor provided by the probe body, the actuator has no capacitive effect on the probe. Moreover, the probe, employing hermetically sealed contacts, has extremely long operating life, and the probe is easily manufactured with comparatively few parts.

Of considerable advantage in the construction of the probe according to the present invention is the absence of required orientation for holder 64. Thus, cylindrical magnet 82 is operative with respect to the reed switches 66 and .8.8 regardless of the orientation of holder 64. Therefore, the probe is rendered easy to assemble and disassemble. For disassembling the probe, bushing 20 is unscrewed, and the internal portions of the probe are longitudinally removed through internal threaded opening 1,8. Thus, holder 64 including the reed switches, as well as resistor `68 and capacitor 70, are withdrawn longitudinally, with plug 44 disengaging the jack formed within extension 42. Reed switches may then be replaced if required, or the resistor 68 and capacitor 70 may be changed, and these components may then be longitudinally reinserted into the probe, with plug 44 being guided within extension 42 at the countersunk end of the extension. As bushing 20 is screwed into the probe, rotation of holder 64 relative to the probe may result, but since the probe configuration and the magnet 82 are symmetrical, the nal rotational position of holder y6'4- and the reed switches is of no importance.

Although the switchable coaxial member according to the present invention is particularly illustrated in an embodiment comprising a probe, it is apparent that a simpler construction not incorporating a probe end is also comprehended according to the broader aspects of the present invention.

While I have shown and described preferred embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. I therefore intend the appended claim to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. A probe for connecting a selected point in electronic circuitry to a measuring instrument comprising:

a conducting cylindrical probe body of substantially non-magnetic material forming an outer coaxial conductor, said probe body having an inner bore,

a probe tip insulatingly supported by and extending from a first end of said probe body,

a cable bushing means received at a second end of said probe body for connecting a coaxial cable to said probe, said bushing means having an outer conductor for connecting the outer conductor of said coaxial cable to the probe body and having an inner conductor for connection to the inner conductor of said coaxial cable,

central conducting means including a reed switch for location within the bore of said probe body, said central conducting means coupling the inner conductor of said cable bushing means to said probe tip,

and an actuator slidably supported on the outes surface of said probe body wherein said actuator carries a permanent magnet therewithin for actuating said reed switch as said magnet moves into operating yjuxtaposition with said reed switch, said magnet providing magnetic flux through said probe body for operating said reed switch.

2. A probe according to claim 1 wherein said central conducting means and the bore of said probe body are dimensioned to provide a characteristic impedance of the same order of magnitude as that of said coaxial cable and said bushing means.

3. A probe for connecting a selected point in electronic circuitry to a measuring instrument comprising:

a conducting sylindrical probe body forming an outer lcoaxial conductor, said probe body having an inner ore,

a probe tip insulatingly supported by and extending from a rst end of said probe body,

a cable bushing received in a second end of said probe body, said bushing having an outer conductor connected to the probe body and having an inner conductor positioned within the outer conductor of said bushing, central conducting means serially including a reed switch for location within the bore of said probe body and a resistor in parallel with said reed switch, said central conducting means connecting the inner conductor of said cable bushing to said probe tip,

an insulating holder slidably received within the bore of said probe body for supporting said reed switch,

and a cylindrical actuator slidable axially on the outer surface of said probe body wherein said actuator carries a cylindrical magnet concentrically therewithin ifor actuating said reed switch as said actuator moves said magnet into operating juxtaposition with said reed switch.

4. The probe according to claim 3 wherein the cable bushing and the probe body at its second end are threaded for matching interengagement.

5. The probe according to claim 3 further including a second reed switch supported by said holder having end conductors adapted for remote connection to said measuring instrument.

16. The probe according to claim -3 wherein said probe tip is provided with jack means and wherein said central conducting means is provided with plug means for selectively connecting tosaid jack means and disengageable therefrom as said bushing is removed from said probe body for removing said reed switch and the holder therefor from the second end of said probe body.

7. The probe according to claim 3 further including a capacitor connected in parallel with said reed switch and said resistor.

'8. The probe according to claim 7 wherein said resistor and said capacitor are physically located between said holder and said probe tip.r

9. The probe according to claim 3 wherein said cylindrical actuator is formed of plastic material.

10. The probe according to claim 3 wherein said probe body is provided with a cylindrical forward body cover formed of insulating material closely received upon the forward end of said probe body adjacent said probe tip,

said probe body also being provided with a rearward body cover formed of insulating material located at the cable bushing end of said probe body, said cylindrical actuator being slidable axially substantially between said forward body cover and said rearward body cover.

11. The probe according to claim 3 wherein said cylindrical probe body is formed with a pair of axially spaced detents located on the outer surface thereof, said actuator including a latching element for selectively engaging the detents in said probe body as said cylindrical magnet is moved into and out of actuating relation with said reed switch.

References Cited UNITED STATES PATENTS 2,790,144 4/ 1957 Sicho 32A-72.5 2,849,681 8/1958 Belart 324-149 3.087,125 4/1963` Scholeeld y.. 335-5X 3,202,784 8/196-5 Santangeli 335-5 3,281,734 1'0/1966 Ansley 20G-19X 3,283,248 11/1966 White 324-725 3,317,830 5/ 1967 Winningstad 324-725 RUDOLPH V. RO-LIN-EC, Primary IExaminer IR. I. CORCORAN, Assistant Examiner U.S. C1. X.R. 335-5, 153 

